The invention relates to a quantum well device, and more particularly to a two-dimensional electron gas field effect transistor (2DEGFET) including an improved InGaAs channel layer parallel to an interface of heterojunction.
A high speed performance is most important to semiconductor devices, because semiconductor devices possessing higher speed performances are permissive of realizing more excellent computers and various high quality micro electronics devices. The realization of the enlargement of the speed of the device performance is mainly provided by following two ways. One is the minimization of the device size, especially the channel length in the field effect transistors. Another is the enlargement of tile electron drift speed moving in a semiconductor material. The improvements in the semiconductor fabrication process techniques makes the gate length short, thereby providing a high speed performance to the transistor. However, the possible increase of the transistor performance speed up to a maximum requires not only the minimization of the device size but the enlargement of the electron drift speed.
The electron drift speed is associated with the mobility of electrons serving as carriers in a semiconductor material. The mobility of the electrons serving as carriers depends upon the effective mass of electrons and the scattering probability. The scattering probability of electrons serving as carriers also depends upon the mean free path defined by the average distance traveled between successive collisions. The effective mass of electrons and the scattering probability and thus the mean free path are determined by the crystal structure possessed by a crystal medium in a semiconductor material. Thus, the electron mobility is determined by the crystal structure possessed by a crystal medium in a semiconductor material. The enlargement of the electron mobility provides high speed performances for semiconductor devices such as field effect transistors.
Generally, the gallium arsenide (GaAs) based crystal medium is likely to possess more excellent electron transport properties as compared to the silicon (Si) based crystal medium, because the electron effective mass in the gallium arsenide (GaAs) based crystal medium is smaller than that in the silicon (Si) based crystal medium. The small electron effective mass allows electrons to be subjected to a large acceleration by an electric field during the travel between the collusions.
Further, electrons in a semiconductor are subjected to scattering by not only lattice oscillations of crystals caused by a thermal energy but ionized impurities doped into the semiconductors. The gallium arsenide (GaAs) based crystal medium possesses a lower scattering probability of electrons as compared to the silicon (Si) based crystal medium. This also contributes towards the promotion of the enlargement of the electron mobility. Thus, the gallium arsenide (GaAs) based crystal medium possesses a larger mean free path than that of the silicon (Si) based crystal medium. Electrons serving as carriers in the gallium arsenide (GaAs) based crystal medium are accelerated without being subjected to scattering by an electric field but for a longer time between the collisions thereby resulting in high speed electrons.
By the way, in view of the minimization of the device size and the promotion of the performance speed of The device, high quality quantum effect devices utilizing quantum effects are attractive. Some of the quantum effect devices have potential well, or quantum well in which electrons are confined thereby the electrons confined in the quantum well are likely to express various quantum effects, where potential barriers fence the potential wells, or quantum well. The one-dimensional electron confinement in the quantum well, or one-dimensional potential well is permissive to promote the enlargement of the electron mobility. The electrons confined in the quantum well, or the one-dimensional potential well are able to move freely in the other two dimensions thereby forming a two-dimensional electron gas. Such a two-dimensional electron gas forms a planar channel layer.
The modulation-doped field effect transistor (MODFET) is one of such semiconductor devices utilizing the two-dimensional electron gas forming a conduction channel region. The accomplishment of modulation doping is to separate electrons serving as carriers from ionized impurities so that electrons are able to possess a large mobility due to no affection by impurity scattering. The modulation doping is realized in a semiconductor heterostructure in which only part of the semiconductor material having a larger energy band gap is doped with the n-type dopant. The other semiconductor material is undoped or unintentionally doped.
With respect to the energy bad gap, the conduction band edge of the semiconductor having the smaller band gap exists below the conduction band edge of the semiconductor having the larger band gap. Electrons move from the large band gap semiconductor into the small band gap semiconductor but in the vicinity of the interface between the heterojunctions, after which the electrons remain in the small energy band gap semiconductor thereby forming a two-dimensional electron gas. The large energy band gap semiconductor serves as the potential barrier to the electrons. The small energy band gap semiconductor serves as the potential well. The electrons are, therefore, confined in the one-dimensional potential well of the small energy band gap planar semiconductor by the fence of the potential barrier provided by the large energy band gap semiconductor.
The property of the two-dimensional electron gas is important to devices such as the modulation-doped field effect transistors (MODFETs) or the high electron mobility transistors (HEMT). One of the most important properties of the two-dimensional electron gas field effect transistors is the electron mobility, especially the mobility of electrons existing in the two-dimensional potential well, or the quantum well serving as the planar channel layer. The mobility of electrons in the two-dimensional potential well is associated with the heterostructure interface between the compound semiconductors forming the potential well and the potential barrier respectively. Other of the most important properties of the two-dimensional electron gas field effect transistors is the sheet electron density in the one-dimensional potential well, or the quantum well serving as the planner channel layer. The sheet electron density in the one-dimensional potential well, or the quantum well serving as the planar channel layer also depends upon the heterostructure interface between the compound semiconductors forming the potential well and the potential barrier respectively. Thus, the property of the one-dimensional electron confinement and the characteristic of the electron transport in other two directions depend upon the semiconductor heterostructures, or the compound semiconductors.
The gallium arsenide / aluminium gallium arsenide (GaAs/AlGaAs) heterostructure is applicable to the heterostructure device such as the modulation-doped field effect transistor, which has been studied previously to other heterostructures. The indium gallium arsenide / aluminium gallium arsenide (InGaAs/AlGaAs) heterostructure is also applicable to the heterostructure device such as the modulation-doped field effect transistor. The InGaAs/AIGaAs modulation-doped field effect transistor is more attractive due to the greater ability at the one-dimensional electron confinement and the excellent property of the electron transport in other two directions parallel to the interface of the heterojunctions. One of the InGaAs/AlGaAs modulation-doped field effect transistor is disclosed in IEEE Electron Devise Letters, vol. EDL-7, No. 12, pp. 444-446, December 1986 reported by T. HENDERSON at al.
One of the conventional InGaAs/AlGaAs two-dimensional electron gas field effect transistor will now be described in detail with reference to FIGS. 1A to 1C to understand concretely the device structure and performance.
With reference to FIG. 1A, a semi-insulating substrate 1 is prepared to form multi-epitaxial layers of compound semiconductors by using molecular beam epitaxy (MBE). The semi-insulating substrate 1 is made of gallium arsenide (GaAs) of the binary compound semiconductor. A buffer layer 2 is formed on the semi-insulating GaAs substrate 1. The buffer layer 2 is made of undoped gallium arsenide (GaAs) of the intrinsic binary compound semiconductors. A planar channel layer 3 is deposited on the undoped binary compound GaAs buffer layer 2. The planar channel layer 3 is made of undoped indium gallium arsenide (InGaAs) of the intrinsic ternary compound semiconductors in which a compound ratio of indium (In) to gallium (Ga) is 0.15:0.85, and thus the fraction of In is 0.15, the fraction of Ga is 0.85. A potential barrier layer 4 is formed on the undoped ternary compound InGaAs planar channel layer 3. The potential barrier layer 4 is made of aluminum gallium arsenide(AlGaAs) of the ternary compound semiconductors in which a compound ratio of aluminium (Al) to gallium (Ga) is 0.15:0.85, and thus the fraction of Al is 0.15, the fraction of Ga is 0.85. The ternary compound AlGaAs semiconductors of the potential barrier layer 4 are doped with the n-type dopant. A cap layer 5 is formed on the n-type ternary compound AlGaAs potential barrier layer 4. The cap layer 5 is made of gallium arsenide (GaAe) of binary compound semiconductor which is heavily doped with n-type dopant.
A source contact 6 made of a conductive material is formed on the n-type binary compound GaAs cap layer 5 by using the evaporation, followed by the allowing process thereby resulting in a formation of ohmic contact between the source contact 6 and the undoped ternary compound InGaAs planar channel layer 3. The n-type binary compound GaAs cap layer 5 underlying the source contact 6 serves as a source region. The doping concentrations of the n-type AlGaAs potential barrier layer 4 and the n-type binary compound GaAs cap layer 5 are so determined as to accomplish the ohmic contact between the source contact 6 and the undoped ternary compound InGaAs channel layer 3. But, the dopant concentration of the n-type binary compound GaAs cap layer 5 is higher than that of the n-type ternary compound AlGaAs potential barrier layer 4. For example, the dopant concentration of the n-type binary compound GaAs cap layers 5 is so determined that the binary compound GaAs semiconductor is likely to take the degenerate state.
Further, a drain contact 8 made of a conductive material is formed on the n-type binary compound GaAs cap layer 5 by using the evaporation, followed by the allowing process thereby resulting in a formation of ohmic contact between the drain contact 8 and the undoped ternary compound InGaAs planar channel layer 3. The n-type binary compound GaAs cap layer serves as a drain region. The doping concentrations of the n-type ternary compound AlGaAs potential barrier layer 4 and the n-type binary compound GaAs cap layer 5 are so determined as to accomplish the ohmic contact between the drain contact 8 and the undoped ternary compound InGaAs planar channel layer 3. But, the dopant concentration of the n-type binary compound GaAs cap layer 5 is higher than that of the n-type ternary compound AlGaAs potential barrier layer 4. For example, the dopant concentration of the n-type binary compound GaAs cap layers 5 is so determined that the binary compound GaAs semiconductor is likely to to take the degenerate state. A recess is formed by etching the n-type binary compound GaAs cap layer 5 between the source electrode 6 and the drain electrode 8. A surface of the n-type ternary compound AlGaAs potential barrier layer 4 is exposed in the recess portion.
A gate contact 7 made of a conductive material is formed on the exposed surface of the n-type ternary compound AlGaAs potential barrier layer 4 at the recess portion between the n-type binary compound GaAs cap layers 5 thereby resulting in a formation of schottky barrier contact. The dopant concentration of the n-type ternary compound AlGaAs potential barrier layer 4 is so determined as to form the schottky barrier contact.
FIG. 1B illustrates distributions of both the Al fraction of the n-type ternary compound AlGaAs potential barrier layer 4 and the In fraction of the undoped ternary compound InGaAs planar channel layer 3 in a perpendicular direction to the heterojunction interface.
FIG. 1C illustrates the energy band gap, or the conduction band edge of the n-type ternary compound AlGaAs potential barrier layer 4, the undoped ternary compound InGaAs planar channel layer 3 and the undoped binary compound GaAs buffer layer 2 in the perpendicular direction to the heterojunction interface as well as the distribution of the electron density of the two-dimensional electron gas confined in the one-dimensional potential well, or the quantum well.
The n-type ternary compound AlGaAs potential barrier layer 4 has the wider energy band gap than the energy band gap of the undoped ternary compound InGaAs planar channel layer 3. Thus, the conduction bad edge of the n-type ternary compound AlGaAs potential barrier layer 4 lies above the conduction band edge of the undoped ternary compound InGaAs planar channel layer 3. The conduction band edge of the n-type ternary compound AlGaAs potential barrier layer 4 has a peak at the heterojunction interface to the schottky gate 7 due to the schottky barrier contact. Conduction band electrons in the n-type ternary compound AlGaAs potential barrier layer 4 are likely to have larger potential energies than the potential energies in the undoped ternary compound InGaAs planar channel layer 3.
Further, the undoped binary compound GaAs buffer layer 2 has the wider energy band gap than the energy band gap of the undoped ternary compound InGaAs planar channel layer 3. Thus, the conduction band edge of the undoped binary compound GaAs buffer layer 2 lies above the conduction band edge of the undoped ternary compound InGaAs planar channel layer 3. The conduction band edge of the undoped binary compound GaAs buffer layer 2 has the uniformity in the level.
Electrons in the conduction band of the n-type ternary compound AlGaAs potential barrier layer 4 move across the heterojunction interface into the undoped ternary compound InGaAs planar channel layer 3. The electrons are confined in the undoped ternary compound InGaAs planar channel layer 3, because the conduction band edge of the the undoped ternary compound InGaAs planar channel layer 3 lies below the conduction band edges of both the n-type ternary compound AlGaAs potential barrier layer 4 and the undoped binary compound GaAs buffer layer 2. Therefore, the undoped ternary compound InGaAs planar channel layer 3 serves as the one-dimensional potential well, or the quantum well, and thus accomplishes the one-dimensional electron confinement in cooperation with the both potential barriers of the n-type ternary compound AlGaAs potential barrier layer 4 and the undoped binary compound GaAs buffer layer 2. The one-dimensional electron confinement in the quantum well forms the two-dimensional electron gas in which the electrons are confined in the one-dimension, for example, in the perpendicular direction to the heterojunction interface, but are able to move freely in the other two-dimensions, for example, in the parallel direction to the heterojunction interface.
Subsequently, the mobility and the sheet electron density of the electrons confined in the quantum well are considered, because the electron mobility and the sheet electron density are most important to the high speed performance of the two-dimensional electron gas field effect transistors.
With respect to the sheet electron density, the deep potential well, or the deep quantum well is permissive of the accomplishment of the strong one-dimensional electron confinement, but the electrons confined in the potential well are able to move freely in the other two-dimensions. The strong one-dimensional electron confinement enables the gain of the sheet electron density. Thus, the deep potential well, or the deep quantum well is able to provide the high sheet electron density to the two-dimensional electron gas confined in the quantum well.
The sheet electron density of the two-dimensional electron gas confined in the quantum well are also associated with the quantum well size which corresponds to the thickness of the undoped ternary compound InGaAs planar channel layer 3. The one-dimensional electron confinement in the quantum well excites a plurality of subbands of the quantum well, concurrently causes the quantization of the electron energies. The each subband energy is inversely proportional to the square of the quantum well size, and thus the thickness of the undoped ternary compound InGaAs planar channel layer 3. Then, the confinement energy of electrons occupying the each energy subband in the quantum well is determined by the difference between the conduction band discontinuity at the hetero-interface and the sub-band energy in the quantum well. If the quantum well size, and thus the thickness of the undoped ternary compound InGaAs planar channel layer 3 is small, the confinement energy of electrons is decreased. Then, the reduction of the quantum well size, and thus the thickness of the undoped ternary compound InGaAs planar channel layer 3 makes the electron confinement difficult. This also makes the sheet electron density low. In contrast, the large quantum well size, and thus the large thickness of the undoped ternary compound InGaAs planar channel layer 3 contributes the high sheet electron density. The enlargement of the quantum well size, and thus the thickness of the undoped ternary compound InGaAs planar channel layer 3 is desirable to secure a high value of the sheet electron density, provided that the above quantum size effects are appeared.
Eventually, in view of the sheet electron density, the deep and large size potential well is desirable to secure the high sheet electron density. The fulfillment of the desirable deep and large size potential well depends upon the crystal medium of the quantum well, and thus the ternary compound In.sub.X Ga.sub.1-X As, for example, the fraction X of In (indium). The gain of the fraction X of In makes the conduction band edge of the energy band low, and thus the depth of the quantum well large.
With respect to the electron mobility, an increase in the faction X of In in the In.sub.X Ga.sub.1-X As channel layer is also permissive of having the electron mobility large. The variation of the fraction X of the In causes the change of the crystal structure of the ternary compound InGaAs. The electron mobility depends upon both the effective mass of electrons and the scattering probability of electrons by the crystal lattice oscillations and especially ionized impurities. Both the electron effective mass and the scattering probability are associated with the crystal structure of the ternary compound InGaAs of the planar channel layer 3 serving as the quantum well. The gain of the fraction X of In (indium) in the ternary compound InGaAs makes the electron effective mass small. Thus, the gain of the fraction X of In (indium) in the ternary compound InGaAs is permissive of having the electron mobility large. Therefore, the conclusion is that the gain of the fraction X of In (indium) in the ternary compound InGaAs of the planar channel layer 3 serving as the quantum well is considerable to secure the great electron mobility and the large sheet electron density, both of which realize the excellent high speed or high frequency semiconductor device.
Although the large fraction X of In (indium) in the ternary compound InGaAs provides the above desirable effects in the gains of the electron mobility and of the sheet electron density, it also provides undesirable results. Disadvantages caused by the large fraction X of In (indium) in the ternary compound InGaAs are subsequently described.
Basically, the heterostructure interfaces of the epitaxial compound semiconductor multilayers are engaged with the problem in the lattice mismatch. The lattice mismatch is caused by the difference in the lattice constants. InGaAs and GaAs respectively possess specific lattice constants, which are different each other. Thus, the realization of the perfect lattice match is extremely difficult. The lattice mismatch intends to cause the misfit dislocations in the crystal structure. The misfit dislocations provides various undesirable affections to the property of the two-dimensional electron gas. The gain of the fraction of indium in the ternary compound InGaAs planar channel layer 3 makes the lattice mismatch distinguished thereby causing the many misfit dislocations. The ternary compound InGaAs planar channel layer 3 including the many misfit dislocations no longer serves as the quantum well planar channel layer. Thus, the two-dimensional electron gas confined in the quantum well of the ternary compound InGaAs planar channel layer 3 including the many misfit dislocations no longer possesses the excellent property in the electron mobility and the sheet electron density. Therefore, the two-dimensional electron gas semiconductor devices including the ternary compound InGaAs planar channel layer 3 including the many misfit dislocations no longer possesses the excellent property in the high speed and high frequency performances.
The generation of the misfit dislocations in the crystal is associated with the thickness of the crystal medium layer. Thus, the generation of the misfit dislocation, and thus lattice mismatch depends upon the thickness of the ternary compound InGaAs planar channel layer 3. The misfit dislocations are likely to be generated on condition that the ternary compound InGaAs planar channel layer 3 has a larger thickness than a critical thickness. The suppression of the generation of the misfit dislocations are achievable, when the ternary compound InGaAs planar channel layer 3 has a smaller thickness than the critical thickness. It is thus required that the ternary compound InGaAs planar channel layer 3 has a smaller thickness than the critical thickness. The critical thickness of the misfit dislocation depends upon the fraction of indium in the ternary compound InGaAs of the planar channel layer 3. The gain of the fraction of indium in the ternary compound InGaAs of the planar channel layer 3 makes the critical thickness small. This forces the ternary compound InGaAs of the planar channel layer 3 to limit the thickness into the small critical thickness or less. As described the above, the small thickness of the ternary compound InGaAs of the planar channel layer 3 makes the one-dimensional electron confinement difficult, thereby resulting in the reduction of the sheet electron density. The detail descriptions are omitted, because it is included in the above recitations.
Eventually, the problems with the conventional two-dimensional electron gas field effect transistors are as follows in brief. The realization of the two-dimensional electron gas field effect transistors possessing the high quality performance, and thus the high speed and high frequency performance requires the gain of the fraction of In (indium) in the ternary compound InGaAs planar channel layer 3. However, the gain of the fraction of indium provides the reduction of the critical thickness of the misfit dislocations caused by the large lattice mismatch. The suppression of the generation of the misfit dislocations forces the channel layer thickness to be smaller than the critical thickness, thereby resulting in the smaller channel layer thickness. The smaller channel layer thickness makes the electron confinement difficult thereby resulting in the reduction of the sheet electron density.
To combat the above problems, it is required to develop a novel two-dimensional electron gas field effect transistor including an improved ternary compound InGaAs planar channel layer in which the two-dimensional electron gas possesses a high electron transport property and a high sheet electron density.